An alignment layer for aligning liquid crystals is important for keeping the order of alignment of the liquid crystals and realizing optical characteristics based on refractive index anisotropy of liquid crystal molecules, and is an essential compositional member that constitutes a liquid crystal display element. Alignment of the liquid crystals significantly affects display characteristics of liquid crystal display elements, and thus various methods for aligning the liquid crystal have been investigated. The liquid crystal display elements can be broadly classified into two types, that is, a vertical alignment type and a horizontal alignment type.
A liquid crystal display device (sometimes referred to as a VA mode liquid crystal display device) using a liquid crystal layer of a vertical alignment type has been widely used in displays for their excellent display characteristics such as high contrast. However, since it cannot be said that the liquid crystal display device using a liquid crystal layer of a vertical alignment type necessarily has sufficient viewing angle characteristics, various methods have been investigated to improve the viewing angle characteristics. As a method for improving the viewing angle characteristics, a multi-domain vertical alignment mode (MVA mode) (incorporating an alignment division structure therein) has become prevalent, which forms a plurality of liquid crystal domains having different alignment directions in one pixel. In the MVA mode, it is necessary to control the tilt alignment of the liquid crystal molecules in order to form the alignment division structure, and as such a method, a method in which a slit (opening) or a rib (projection structure) is provided in electrodes, is used. However, with the use of the slit or the rib, the slit or the rib is linear unlike a case where a pretilt direction is defined by inserting liquid crystal molecules into two alignment films used in a TN mode used in the related art, and thus, the ability to control the alignment for the liquid crystal molecules becomes uneven within a pixel, whereby a problem of generation of a distribution in the response speeds arises.
In addition, there is another problem that a region provided with a slit or a rib exhibits decreased optical transmittance, resulting in a decrease in display luminance.
As another method for controlling the tilt alignment, there is disclosed a polymer alignment support (PSA; Polymer Sustained Alignment) technology in which photo- or thermo-polymerizable monomers are incorporated into a liquid crystal, the monomers being polymerized while tilting the liquid crystal molecules by voltage application so that the tilt direction of the liquid crystal molecules is memorized (see PTL 1). This method can overcome the problem in the distribution of the response speeds or a decrease in the optical transmittance in the slit-and-rib method. However, this method faces a problem such as changes in characteristics caused by the addition of monomers in the liquid crystal material, difficulty in controlling the process, and adverse effects of the residual monomers.
In order to avoid these problems, it is preferable even for the VA mode liquid crystal display device to form an alignment division structure by controlling the tilt alignment with an alignment film. As a method of providing the vertical alignment film with an ability to control the tilt alignment, there is a rubbing method. In this method, an alignment film made of a polyimide or the like is applied onto a substrate, and then the alignment film is rubbed with rubbing cloth to control the alignment direction and the pretilt angle. However, it is difficult to form a precise alignment division structure by the rubbing method, and thus problems of static electricity caused by friction and generation of impurities arise.
Meanwhile, as one of liquid crystal display devices using a liquid crystal layer of the horizontal alignment type, there is an In Plane Switching (IPS) mode liquid crystal display device. The IPS mode liquid crystal display device has little dependency on viewing angles such as in contrast and color tone, and is widely used in displays due to its excellent display characteristics. In the IPS mode, in order to reduce viewing angle dependency in the black display and the color reproducibility, it is required to have a low pretilt angle of one degree or less on the electrode surface. Even when achieving the horizontal alignment, a rubbing method as a general alignment method is used. However, when a horizontal alignment treatment is carried out by a rubbing treatment with a polyimide alignment film, the pretilt angle provided to the liquid crystal molecules exceeds one degree, and thus, a problem that the display characteristics are deteriorated arises.
From these problems, in any alignment mode of the vertical alignment type and the horizontal alignment type, it is important to control the alignment direction and the pretilt angle using the alignment film so as to improve the display characteristics. As a method for controlling the tilt alignment with an alignment film, a photo-alignment method is known, in addition to the methods using rubbing treatment (see PTL 2). In the photo-alignment method, a precise alignment division structure can be formed easily by changing the illumination pattern of light, and static electricity or generation of impurities is difficult to occur, as compared with the rubbing treatment since a non-contact treatment on the alignment film can be carried out, and thus, it is expected to solve the above-described problems and to improve the display characteristics.
As the materials which can be a photo-alignment layer for the liquid crystal display element, a compound having a photochemically isomerizable site, such as an azobenzene derivative (see PTL 3), a compound having a photochemically crosslinkable site, such as a cinnamic acid derivative, a coumarin derivative, and a chalcone derivative (see PTLs 4, 5, and 6), a compound causing an anisotropic photo-degradation, such as a polyimide derivative, and the like are known.
However, the photo-alignment method using these compounds has a problem such as a low voltage holding ratio (VHR), as compared with a case using an ordinary alignment film. It also has problems such as a high residual voltage (RDC) caused by residual charges and frequent occurrence of seizure.
Therefore, various characteristics such as reliability, which allows realization of performance for controlling the tilt alignment of the liquid crystals and use in active matrix driving, are required, and photo-alignment layers for liquid crystals, which satisfy the requirements, have been required.
As described above, there has been a demand for a liquid crystal alignment layer having a superior ability to control the alignment of the liquid crystals and the pretilt angles and further, a high voltage holding ratio (VHR). There has been a further demand for a liquid crystal alignment layer having a superior ability to control the alignment of the liquid crystals and the pretilt angles, a high voltage holding ratio (VHR), low residual voltage, and infrequent occurrence of seizure.
Furthermore, the photo-alignment method is expected to be commercialized since it has excellent mass productivity and an ability of coping with a large substrate.
The material to be used as a material for the photo-alignment layer for a liquid crystal display element or an optical anisotropic body, a compound having a photochemically isomerizable site, such as an azobenzene derivative, has been known (see PTL 3). As an important characteristic in the use of the photo-alignment layer, what is important is a degree of irradiation dose of anisotropic light with which realignment will be performed (hereinafter referred to as sensitivity), and the photo-alignment layer using a compound having an azo group is excellent in the sensitivity and thus, exhibits an ability of aligning liquid crystals at a low irradiation dose of approximately 500 mJ/cm2.
However, since the photo-alignment layer using a compound having an azo group is a low-molecular-weight compound, there may occur problems in a step of producing a liquid crystal cell, for example, it is flown off in a washing step or eroded by an adhesive member such as a sealing agent, in some cases. Further, in the case of producing an optical anisotropic body formed by repeating the lamination of a photo-alignment layer and a polymerizable liquid crystal layer in the production of an optical anisotropic body, the process includes a step of coating a polymerizable liquid crystal composition solution onto a photo-alignment layer, or a step of coating a composition solution for a photo-alignment film onto a polymerizable liquid crystal layer, but there have been cases where the liquid crystal alignment layer or the polymerizable liquid crystal layer after the production has been affected, and thus the film has been peeled, or uniform optical characteristics could not be obtained, by a solvent used in the coating solution, or the like.
Moreover, as a material which can be used for a photo-alignment layer, a compound having a photochemically crosslinkable site, such as a cinnamic acid derivative, a coumarin derivative, and a chalcone derivative; a compound causing an anisotropic photo-degradation, such as a polyimide derivative; and the like have been known (see PTLs 4, 6, and 7). However, the photo-alignment layer formed with a compound having a photochemically crosslinkable site, such as a cinnamic acid derivative, is poor at sensitivity, as compared with a compound having an azo group, is required to be irradiated with light having a potent anisotropic property during the photo-alignment, and thus, is not easily commercialized due to large-scale devices needed.
On the other hand, there has already been disclosed a polymer compound which has a skeleton having an azo group as a skeleton having a photochemically isomerizable site and having a cinnamic acid skeleton as a skeleton having a photochemically crosslinkable site (see PTL 8). However, in the cited document, the polymer compound is used as an optical non-linear polymer, there is no suggestion of use as an optical anisotropic body, and it was not clear how to use specifically and what kind of characteristics the compound has.
As described above, there has been a demand for development of a material for a photo-alignment layer, which is highly sensitive and is not eroded by a solvent or the like.